Identification
A long white day is followed by a black day that gaps down and is completely engulfed by the real body of the first day.

The Psychology
In an uptrend or within a bounce of a downtrend a long white day occurs. The next day's gap down comes as a surprise to bulls who thought they were sitting on a great position the previous day. Reliability of the bearish Harami is low, so a weak following day is needed for confirmation.

Bearish Harami formations carry low reliability but when they occur at resistance like with APH, reliability increases and the risk/reward tradeoff shifts to a more favorable level.

Here is an example of a bearish Harami forming right at resistance. This situation offers the safest entry on the short side because your risk to the upside is small while your potential gain is great because the stock will likely trade back to support.

AA also formed a bearish Harami at resistance. Notice the lack of volume on the rally up to the previous high. That's hint #1 that the stock has limited upside potential. When the stock got rejected and dropped on big volume and then gapped down the next day, the Harami candle was confirmed, and the stock did indeed reverse course.